curate API reference content into Swift Package Manager docs

[heckj]

Page URL

https://docs.swift.org/latest/documentation/packagemanagerdocs

Description

Following up from PR #133, which suppressed the PackageDescription and PackagePlugin API reference documentation from the combined collections, work to embed the content (or links to the content) for those reference API collections.

The symbols are actively included in the combined documentation at the tail end of this process, but I think in order to work around DocC' combined limitations where, we'll need to create a slightly custom build setup:

• build and generate all the relevant symbol graphs for these APIs
• when we build the packageManagerDocs collection, include references to those additional symbol graph collections
• replicate the curation and content from their DocC catalogs into the catalog with PackageManagerDocs to "embed" that content as API reference within it. The standard library has a precedent of doing this using an "API Topic Group", which presents as a heading with disclosure affordance that expands to let you drill down and view into the content from a human navigation perspective.